A sunspot viewed close-up in ultraviolet light, taken by the TRACE spacecraft.
Credit: Wikimedia
SYDNEY: A new technique for detecting sunspots days before they erupt has been developed to provide better space weather forecasts.
Sunspots are large, dark features on the solar surface that appear when the Sun's magnetic field rises up from the interior. They have been studied for the past 400 years, but scientists have never been able to observe their activity deep below the surface, so it was impossible to know when and where they would erupt.
Now, a team of scientists from the W.W.Hansen Experimental Laboratory at Stanford University in California have devised a method for detecting subsurface magnetic fields days before they emerge, which will have implications for telecommunications on Earth, and could help solar physicists to unlock the mystery of how the Sun works.
"It is very important that we are able to predict these events. With our technique, we can detect sunspots one to two days before they appear on the solar disc, and we couldn't do that before," said physicist Stathis Ilonidis, lead author of the paper published today in Science.
"These are regions where major eruptive events such as flares and coronal mass ejections occur, so this event can change the conditions of the space weather, causing black-outs and interrupting GPS and navigation services [on Earth] and posing hazards for astronauts in space craft."
Speeding up at sunspots
The analysis technique is known as time-distance helioseismology, and is similar to an approach widely used in earthquake studies. The method is based on an analysis of vibrations on the solar surface and has been in development for the past 20 years, but until now has not been adequate for sunspot detection.
The technique involves measuring the travel times of acoustic waves that travel along the subsurface magnetic field between two points on the solar surface. These waves can be observed using a highly sensitive instrument known as the Helioseismic and Magnetic Imager on board NASA's Solar Dynamics Observatory (SDO).
The researchers found that when the acoustic waves encounter a sunspot on their path between the two points (at a distance of 100,000 to 200,000 km apart), it will speed up, shaving 12 to 16 seconds of a total travel time of one hour.
Travel time map
By selecting millions of points and measuring millions of travel times between these points, the researchers were able to create a travel time map showing at which locations inside the Sun the acoustic waves travel faster or slower.
"We try to identify locations where the travel time is significantly shorter, which indicates that there is a sunspot region there," said Ilonidis. "A lot of people have tried to detect sunspots before they appear on the disc using this technique, the difference now is that we are using a new measurement scheme."
"It's not a completely new result, but what they have done is better," commented Martin Asplund from the Australian National University's College of Physical and Mathematical Sciences in Canberra. "It's crucial to be able to detect things that are deeper in the solar interior."
